The present invention relates to a solid polymer electrolyte membrane used in fuel cells, water electrolysis, hydrogen halide acid electrolysis, sodium chloride electrolysis, oxygen concentrators, moisture sensors, gas sensors, etc.; an electrocatalyst-coating solution; an assembly of said membrane and electrodes; and fuel cells.
Solid polymer electrolytes are solid polymeric materials having groups characteristic of the electrolytes (e.g. sulfonic acid groups) in the polymer chain. Since they bind strongly to specific ions or are selectively permeable to cations or anions, they are utilized for various purposes after being molded or shaped into particles, fiber or a membrane. For example, they are utilized in electrodialysis, diffuse dialysis, diaphragms for cell, etc.
In a reformed-gas fuel cell, an electromotive force is obtained by providing a pair of electrodes on both sides, respectively, of a proton-conductive solid polymer electrolyte membrane, supplying hydrogen gas obtained by reforming a low-molecular weight hydrocarbon such as methane, methanol or the like to one of the electrodes (a hydrogen electrode) as a fuel gas, and supplying oxygen gas or air to the other electrode (an oxygen electrode) as an oxidizing agent. In water electrolysis, hydrogen and oxygen are produced by electrolyzing water by the use of a solid polymer electrolyte membrane.
As a solid polymer electrolyte membrane for a fuel cell, water electrolysis or the like, fluorine-containing solid polymer electrolyte membranes represented by perfluorocarbon sulfonic acid solid polymer electrolyte membranes with a high proton conductivity known by their trade names of Nafion® (a registered trade name, mfd. by E.I. du Pont de Nemours & Co.), Aciplex® (a registered trade name, ASAHI Chemical Industry Co., Ltd.) and Flemion® (a trade name, mfd. by Asahi Glass Co., Ltd.) are used because of their excellent chemical stability.
In sodium chloride electrolysis, sodium hydroxide, chlorine and hydrogen are produced by electrolyzing an aqueous sodium chloride solution by the use of a solid polymer electrolyte membrane.
In this case, since the solid polymer electrolyte membrane is exposed to chlorine and an aqueous sodium hydroxide solution of high temperature and concentration, a hydrocarbon solid polymer electrolyte membrane having a low resistance to chlorine and the solution cannot be used. Therefore, as a solid polymer electrolyte membrane for sodium chloride electrolysis, there is generally used a perfluorocarbon sulfonic acid solid polymer electrolyte membrane which is resistant to chlorine and the aqueous sodium hydroxide solution of high temperature and concentration and has carboxylic acid groups introduced partly into the membrane surface in order to prevent the reverse diffusion of ions generated.
The fluorine-containing solid polymer electrolyte membranes represented by the perfluorocarbon sulfonic acid solid polymer electrolyte membranes have a very high chemical stability because of their C—F bonds and hence are used not only as a solid polymer electrolyte membrane for the above-mentioned fuel cell, water electrolysis or sodium chloride electrolysis but also as a solid polymer electrolyte membrane for hydrogen halide acid electrolysis. In addition, they are widely utilized in moisture sensors, gas sensors, oxygen concentrators, etc. by taking advantage of their proton conductivity.
The fluorine-containing solid polymer electrolyte membranes, however, are disadvantageous in that they are difficult to produce and are very expensive. Therefore, the fluorine-containing solid polymer electrolyte membranes are used for special purposes, for example, they are used in solid polymer membrane fuel cells for space research or military use. Thus, they have been difficult to use for livelihood in, for example, a solid polymer membrane fuel cell as a low-pollution power source for automobile.
As inexpensive solid polymer electrolyte membranes, the following aromatic hydrocarbon solid polymer electrolyte membranes, for example, have been proposed. JP-A-6-93114 has proposed a sulfonated polyether ether ketone membrane. JP-A-9-245818 and JP-A-11-116679 have proposed sulfonated polyether sulfone membranes. JP-A-11-67224 has proposed a sulfonated polyether ether sulfone membrane. JP-A-10-503788 has proposed a sulfonated acrylonitrile-butadiene-styrene polymer membrane. JP-A-11-510198 has proposed a sulfonated polysulfide membrane. JP-A-11-515040 has proposed a sulfonated polyphenylene membrane.
These aromatic hydrocarbon solid polymer electrolyte membranes obtained by sulfonating engineering plastics are advantageous in that their production is easier and entails a lower cost as compared with the production of the fluorine-containing solid polymer electrolyte membranes represented by Nafion®.
The sulfonated aromatic hydrocarbon solid polymer electrolyte membranes, however, are disadvantageous in that they tend to be deteriorated. According to JP-A-2000-106203, a solid polymer electrolyte membrane having an aromatic hydrocarbon skeleton tends to be deteriorated because hydrogen peroxide produced in a catalyst layer formed on the boundary surface between the solid polymer electrolyte membrane and an oxygen electrode oxidizes and deteriorates the aromatic hydrocarbon skeleton.
Therefore, for example, JP-9-102322 has proposed a sulfonated polystyrene-grafted ethylene-tetrafluoroethylene copolymer (ETFE) membrane comprising a main chain formed by the copolymerization of a fluorocarbon type vinyl monomer and a hydrocarbon type vinyl monomer and hydrocarbon side chains having sulfonic acid groups, as a solid polymer electrolyte membrane which has an oxidative-deterioration resistance equal to or higher than that of the fluorine-containing solid polymer electrolyte membranes and can be produced at a low cost.
The sulfonated polystyrene-grafted ETFE membrane disclosed in JP-A-9-102322 is reported as follows: it is inexpensive, has a sufficient strength as solid polymer electrolyte membrane for a fuel cell, and can be improved in electric conductivity by increasing the amount of sulfonic acid groups introduced.
However, in the sulfonic acid type polystyrene-grafted ETFE membrane, the main chain portion formed by the copolymerization of a fluorinated vinyl monomer and a vinyl monomer has a high resistance to oxidative deterioration, but the side chain portion having sulfonic acid groups introduced thereinto is an aromatic hydrocarbon polymer which is subject to oxidative deterioration. Therefore, said membrane is disadvantageous in that when the membrane is used in a fuel cell, the resistance to oxidative deterioration of the whole membrane is not sufficient, resulting in a low durability.
U.S. Pat. No. 4,012,303 and U.S. Pat. No. 4,605,685 have proposed sulfonic acid type poly(trifluorostyrene)-grafted ETFE membranes obtained by graft-copolymerizing α,β,β-trifluorostyrene onto a membrane produced by the copolymerization of a fluorinated vinyl monomer and a vinyl monomer, and introducing sulfonic acid groups into the α,β,β-trifluorostyrene units to obtain a solid polymer electrolyte membrane.
These membranes are obtained by using α,β,β-trifluorostyrene prepared by partial fluorination of styrene, in place of styrene on the assumption that the chemical stability of the above-mentioned polystyrene side chain portion having sulfonic acid groups introduced thereinto is not sufficient. The synthesis of α,β,β-trifluorostyrene as a starting material for the side chain portion, however, is difficult. Therefore, when said membranes are used as a solid polymer electrolyte membrane for a fuel cell, there is a cost problem as in the case of the above-mentioned Nafion®.
Moreover, α,β,β-trifluorostyrene is disadvantageous in that because of its low polymerizability, the amount of α,β,β-trifluorostyrene introducible as grafted side chains is small, so that the resulting membrane has a low electric conductivity.